1. Field of the Invention
The present invention relates to a smectic liquid crystal and a liquid crystal display using the same. More particularly, the present invention relates to a smectic ferroelectric liquid crystal which enables grayscale display, and a liquid crystal display using the same.
2. Description of the Related Art
A liquid crystal display using a surface stabilized ferroelectric liquid crystal (SSFLC) is proposed as a liquid crystal display in which a wide viewing angle and a quick response can be expected. Such a liquid crystal display is disclosed in xe2x80x9cSubmicrosecond bistable electro-optic switching in liquid crystalsxe2x80x9d, N. A. Clark et al., Appl. Phys. Lett., 36, pp.899-901 (1980). The SSFLC has a spontaneous polarization which is inverted by the application of an external electric field. This inversion causes a direction of a liquid crystal molecule in the SSFLC to be switched. Two states of a bright state and a dark state are generated depending on the direction of the liquid crystal molecule in the SSFLC.
However, the SSFLC is bi-stable. The SSFLC display has the only two states of the bright state and the dark state. Thus, conventionally, the SSFLC liquid crystal display has a problem that the grayscale display is difficult.
A method for driving the SSFLC by using an AC stabilization effect is disclosed in xe2x80x9cA Multiplexed Ferroelectric LCD Using ac Field-Stabilized Statesxe2x80x9d, J. M. Geary, SID 85 Digest, pp.128-130. Also, a SSFLC display using the xcfx84xe2x88x92Vmin drive method is disclosed in xe2x80x9cThe xe2x80x98Joers/Alveyxe2x80x99 Ferroelectric Multiplexing Schemexe2x80x9d, P. W. H.Surguy et al., Ferroelectric, 122, pp.63-79 (1991), xe2x80x9cColor Digital Ferroelectric LCDs for Laptop Applicationsxe2x80x9d, P. W.Ross et al., SID 92 Digest, pp.217-220 (1992), and Japanese Laid Open Patent Application (JP-A-Heisei 9-318921). However, the problem can not be solved by the drive method for driving the SSFLC by using the AC stabilization effect or the xcfx84xe2x88x92Vmin drive method.
Therefore, a dithering method for spatially dividing pixels, a field cutting method of time-divisionally generating graylevels and an area gradation method for controlling a generation degree of a polarization inversion area within a pixel are frequently used for achieving grayscale display. However, those methods have a problem that a circuit for driving a liquid crystal is complex.
Moreover, the conventional SSFLC is hard to be driven by a TFT(Thin Film Transistor) because of its large spontaneous polarization. Thus, the liquid crystal display using the SSFLC should be driven by a simple matrix drive. However, it is difficult for the simple matrix drive to attain the liquid crystal display having a high resolution and a high image quality.
On the other hand, as the liquid crystal display in which grayscale display is achieved, a liquid crystal display which uses anti-ferroelectric liquid crystal material is disclosed in xe2x80x9cFerroelectric Liquid Crystal Display Using Tristable Switchingxe2x80x9d, Y. Yamada et al., Jpn. J. Appl. Phys., 29, pp.1757-1764 (1990) and xe2x80x9cAntiferroeletctric Chiral Smectic Phases Responsible for the Tristable Switching in MHPOBCxe2x80x9d, A. D. L. Chandani et al., Jpn. J. Appl. Phys., 28, pp.L1265-L1268(1989). The anti-ferroelectric liquid crystal material has a tri-stable property. A liquid crystal display making use of switching the anti-ferroelectric liquid crystal material under an application of a bias voltage achieves grayscale display under an application of a bias voltage.
However, the liquid crystal display using the anti-ferroelectric liquid crystal material has a problem that the bias voltage is necessary for the grayscale display and that a drive wave form is complex in a case of a display element having a high resolution and many scan lines. Moreover, in the conventional liquid crystal display using the anti-ferroelectric liquid crystal material, it is difficult to carry out a TFT drive because of a large value of a spontaneous polarization of the anti-ferroelectric liquid crystal material.
Moreover, as a ferroelectric liquid crystal display in which grayscale display can be done, a liquid crystal display device using a deformed helix ferroelectric (DHF) liquid crystal is disclosed in xe2x80x9cBehaviour of ferroelectric smectic liquid crystals in electric fieldxe2x80x9d, Ostrovski et al., Advances in Liquid Crystal Research and Applications, Oxford/Budapest (1980) pp.469-482 and Japanese Laid Open Patent Application (JP-A-Heisei, 1-152430). As shown in FIG. 1, in the liquid crystal display using the DHF liquid crystal, a distance d1 between substrates 101, 102 is set to be longer than a pitch d2 of a helix 104 formed by liquid crystal molecules 103 of the DHF liquid crystal. In the arrangement of the substrate 101, 102 and the liquid crystal molecules 104, the generation of the helix is never suppressed by surface stabilization. As a result, the liquid crystal molecules 103 are arrayed such that the helix is drawn in a direction parallel to the substrates 101, 102.
A diffraction grating is formed when the pitch d2 of the helix 104 is within a wave length range of a light. The diffraction grating is not formed if the pitch d2 of the helix 104 is set to be shorter than the wave length range of the visible light. The pitch d2 of the helix 104 is typically set to be shorter than a half wave length xc2xdxcex. Apparent refractive indexes are averaged if the pitch d2 of the helix 104 is set to be shorter than the wave length range of the light and thereby the diffraction grating is not formed. At this time, the DHF liquid crystal can be treated similarly to a medium having a uniaxial birefringence parallel to a helical axis.
That is, the DHF liquid crystal shows double refraction in a helix axis direction when a voltage is not applied. When the voltage is applied, the DHF liquid crystal is gradually deviated from the helix array in the liquid crystal orientation. Thus, it has the distorted helix structure, which causes a transmissivity to be changed. Hence, the DHF liquid crystal can carry out the continuous grayscale display. Its driving method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 6-194625). Moreover, another DHF liquid crystal is disclosed in Japanese Patent Office Gazette (Jp-B 2532606).
Also, a polymer stabilized FLC (ferroelectric liquid crystal) is known as a ferroelectric liquid crystal material which can attain the continuous grayscale display. The polymer stabilized FLC is disclosed in xe2x80x9cMesogenic Polymer Stabilized Ferroelectric Liquid Crystal Display Exhibiting Monostability with High Contrast Ratio and Grayscale Capabilityxe2x80x9d, H.Furue et al., Jpn. J. Appl. Phys., 36, pp.L1517-L1519 (1997) and xe2x80x9cFabrication of a Zigzag Defect-Free Surface-Stabilized Ferroelectric Liquid Crystal Display Using Polyimide Orientation Filmxe2x80x9d, H. Furue, et al., Jpn. J. Appl. Phys., 37, pp.3417-3421(1998). The polymer stabilized FLC liquid crystal includes monomer together with the FLC (Ferroelectric Liquid Crystal) material. The polymer stabilized FLC is stabilized by irradiating an ultraviolet light while aligning the liquid crystal molecules in one direction through the application of electric field. The continuous grayscale display can be done in the polymer stabilized FLC liquid crystal. FIG. 2 shows a voltage-to-contrast ratio curve in this polymer stabilized FLC measured at four temperatures (25xc2x0 C., 30xc2x0 C., 35xc2x0 C. and 40xc2x0 C.). The contrast ratio is a ratio of the transmissivity to 0V at the darkest time, and substantially corresponds to a voltage transmissivity curve. As shown in FIG. 3, the transmissivity is gradually changed. The continuous grayscale display can be obtained in the polymer stabilized FLC display.
Also, another liquid crystal display which can attain the continuous grayscale display is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 4-212126). In this liquid crystal display, two substrates are arranged such that the orientations are substantially parallel to each other. Projection components to the substrate of a molecule axis of liquid crystal molecules of a ferroelectric liquid crystal having a chiral-smectic C phase and projection components to the substrates in an axis direction of cones drawn by the liquid crystal molecules are respectively equal to orientation process directions. This state is mono-stabilized as an initial state, namely, set at a mono-stabilized state. In this liquid crystal display, the liquid crystal orientation is mono-stabilized on a top surface or a bottom surface on the cone when the voltage is not applied. The orientation is changed, and the transmissivity is increased, depending on the positively or negatively electric field. This change is continuously generated in accordance with the magnitude of the electric field. Thus, the continuous gradation can be attained in this device.
Moreover, another ferroelectric liquid crystal display which can attain the continuous grayscale display is disclosed in xe2x80x9cHalf V-shaped switching mode FLCDxe2x80x9d, M. Terada et al., Extended Abstracts of The 46th Spring Meeting, p.1316 (1999). The ferroelectric liquid crystal display employs a mono-stable FLC having a Half V-shaped switching mode. This mono-stable FLC is formed by performing a Ch-Smc* phase transition while applying a voltage to the FLC material having the Iso. (isotropic phase)xe2x80x94Ch (cholesteric, namely, chiral nematic N*)xe2x80x94SmC* (chiral-smectic C) phase transition. The fact that such a method enables the mono-stabilization is disclosed in xe2x80x9cStructure And Properties of Ferroelectric Liquid Crystalxe2x80x9d, pp. 240-241 (Corona Corporation, 1990). The mono-stable FLC having the Half-V-shaped switching mode has the correspondence in which a brightness is changed only by one polarity of a voltage.
Moreover, a liquid crystal display which can attain the grayscale display by using a chevron bookshelf structure is disclosed in Japanese Patent Office Gazette (Jp-B 2849112) and xe2x80x9cGray shades capability of SSFLCs by using a bookshelf layer structure FLCxe2x80x9d, A. Mochizuki et al., Digests of IDW""96 Vol.1, pp.69-72 (1996). Also, a smectic liquid crystal material in which a curve of a light transmissivity to an application voltage indicates a V-shaped curve having a minimum value at OV is disclosed in xe2x80x9cThresholdless Antiferroelectricity in Liquid Crystalsxe2x80x9d, S.Inui et al., Preliminary Lecture Manuscript of 21-th Liquid Crystal Meeting, p.222-223 (1995), xe2x80x9cAn Antiferroelectronic Liquid Crystal with a Novel Hysteresis loopxe2x80x9d, C. Tanaka et al., Preliminary Lecture Manuscript of 21-th Liquid Crystal Meeting, pp.250-251 (1995), and xe2x80x9cVoltage-Holding Properties of Thresholdless Antiferroelectric Liquid Crystals Driven by Active Matricesxe2x80x9d, T. Saishu et al. , SID 96 Digest, pp.703-706 (1996).
However, the conventional ferroelectric liquid crystal displays which can attain the grayscale display as mentioned above have the large values of the spontaneous polarization. Therefore, the TFT drive is difficult. Thus, it is necessary that the conventional ferroelectric liquid crystal displays which can attain the grayscale display are driven by the simple matrix drive. Hence, it is difficult that the conventional ferroelectric liquid crystal displays which can attain the grayscale display achieve the liquid crystal display with the high resolution and the high image quality. This is because in the simple matrix drive, the number of scan lines is limited and the contrast is limited.
In order to solve those problems, we discloses a liquid crystal material having a low spontaneous polarization in which the TFT drive can be done and a liquid crystal display using the same in Japanese Laid Open Patent Application (JP-A-Heisei, 10-338877).
Moreover, the conventional ferroelectric liquid crystal displays which can attain the conventional grayscale display have a problem of requiring a reset drive and a drive using a calculation between frames. This is because while a voltage is retained in the liquid crystal by an accumulation capacitor since the TFT is turned OFF, its retained voltage is varied by the inversion of the spontaneous polarization to thereby induce a response referred to as a step response. Therefore, the reset drive and the drive using the calculation between the frames are required in order to prevent the step response from being induced.
The reset drive and the drive using the calculation between the frames are described in detail in Japanese Laid Open Patent Application (JP-A-Heisei, 10-41689) and Japanese Laid Open Patent Application (JP-A-Heisei, 10-43839) in relation to the application of the present applicant. Also, Japanese Laid Open Patent Application (JP-A-Heisei, 10-65177) filed by the present applicant discloses a technique for writing one frame or a plurality of frames to one field as a method which does not use the reset drive and the calculation between the frames. However, even this method has a problem that signals having a high frequency are needed in the liquid crystal display. As mentioned above, the reset drive and the drive using the calculation between the frames are required, and the drive frequency is increased.
Therefore, an object of the present invention is to provide a smectic liquid crystal material which can attain a grayscale display and a liquid crystal display using the same;
Another object of the present invention is to provide a smectic liquid crystal material having a low drive voltage which can attain the grayscale display, and a liquid crystal display using the same;
Still another object of the present invention is to provide a smectic liquid crystal material having a low drive frequency which can attain the grayscale display, and a liquid crystal display using the same;
Yet still another object of the present invention is to provide a liquid crystal display having a high image quality which can attain the grayscale display;
It is also an object of the present invention to provide an active matrix type liquid crystal display having a high image quality which can attain the grayscale display;
It is another object of the present invention to provide an active matrix type liquid crystal display which can attain the grayscale display and have a high image quality, a low drive voltage and a low drive frequency; and
It is still another object of the present invention to provide an active matrix type liquid crystal display which can attain the grayscale display without requiring the reset drive and the calculation between the frames.
In order to achieve an aspect of the present invention, a liquid crystal display includes a liquid crystal layer including a liquid crystal molecule. The liquid crystal molecule has a long axis, a short axis perpendicular to the long axis, a spontaneous polarization Ps along the short axis, a first permittivity xcex5// along the long molecular axis, a second permittivity xcex5xe2x8axa5 along the short axis. The second permittivity xcex5xe2x8axa5 is derived from polarizations other than the spontaneous polarization. When a permittivity anisotropy xcex94xcex5 is defined as
xcex94xcex5=xcex5//xe2x88x92xcex5xe2x8axa5,
the permittivity anisotropy xcex94xcex5 has a value of
xcex94xcex5 less than 0.
And an orientation of the liquid crystal molecule is determined by an effective electric field. The effective electric field is a sum of an exterior electric field applied to the liquid crystal layer and a depolarization field generated by the spontaneous polarization.
The liquid crystal display may further include a first pair of electrodes between which the liquid crystal layer is located. The first pair of electrodes and the liquid crystal layer constitute a liquid crystal cell. In this case, the spontaneous polarization Ps is determined as
xcex94V less than Vcc/2n,
where xcex94V is a change in a voltage between the first pair of electrodes caused by a reverse of the spontaneous polarization Ps after charging of the first pair of electrodes to a charging voltage is finished, and Vcc is a maximum driving voltage applied to the first pair of electrodes, and n is a number of graylevels of the liquid crystal display.
The liquid crystal display may further include a storage capacitor. The storage capacitor includes a second pair of electrodes and a insulating layer between the second pair of electrodes and is connected to the liquid crystal cell in parallel. Desirably, the spontaneous polarization Ps has a value of   0   less than       P    s     less than                     ϵ        0            ⁢              (                                            ϵ              lc                                      d              lc                                +                                                    ϵ                sc                            ·              k                                      d              sc                                      )              ·                  V        cc                    2        ⁢        n            
where xcex50 is a permittivity of vacuum, and xcex51c is a permittivity of the liquid crystal layer in a same direction of the external electric field, d1c is a first distance between the pair of electrodes, and xcex5sc is a permittivity of the insulating layer, and dsc is a second distance between the other pair of electrodes, and k has a value of
k=Ssc/S1c
where S1c is an aperture of each of the first pair of electrodes and Ssc is an aperture of each of the second pair of electrodes.
It may be more desirable that the spontaneous polarization Ps has a value of   0   less than       P    s     less than                     ϵ        0            ⁢              (                                            ϵ              lc                                      d              lc                                +                                                    ϵ                sc                            ·              k                                      d              sc                                      )              ·                            V          cc                          4          ⁢          n                    .      
With respect to the liquid crystal display, a first torque applied to the liquid crystal molecule and generated by coupling of a permittivity anisotropy of the liquid crystal molecule to the effective electric field is desirably larger than a second torque applied to the liquid crystal molecule and generated by coupling of the spontaneous polarization to the effective electric field.
The liquid crystal display may further include a first pair of electrodes between which the liquid crystal layer is located. The liquid crystal molecule lies on a surface of a cone having an apical angle of 2xcex8. One of ends of the liquid crystal molecule is fixed on a vertex of the cone and the other end of the liquid crystal is rotated along a circular base of the cone. A perpendicular from the vertex to the circular base is generally parallel to the pair of electrodes. In this case, the permittivity anisotropy xcex94xcex5 desirably has a value of       "LeftBracketingBar"          Δ      ⁢              xe2x80x83            ⁢      ϵ        "RightBracketingBar"     greater than             P      s                                ϵ          0                ·                  (                      E            +                                                            P                  s                                ⁢                cos                ⁢                                  xe2x80x83                                ⁢                φ                                                              ϵ                  0                                ⁢                                  ϵ                  lc                                                              )                ·                  sin          2                    ⁢              θ        ·        cos            ⁢              xe2x80x83            ⁢      φ      
where E is an amplitude of the exterior electric field, xcex51c is a permittivity of the liquid crystal layer in a same direction of the external electric field, xcfx86 is an azimuthal angle accomplished by a first segment and a second segment. The first segment is directed in a radius direction of the circular base and in parallel to the first pair of electrodes. The second segment links the other end of the liquid crystal molecule and a center of the circular base of the cone.
The liquid crystal display may further include a first pair of electrodes between which the liquid crystal layer is located. The liquid crystal layer may have a chevron structure including a plurality of smectic layers. The liquid crystal molecule lies on a surface of a cone having an apical angle of 2xcex8. One of ends of the liquid crystal molecule is fixed on a vertex of the cone and the other end of the liquid crystal is rotated along a circular base of the cone. A perpendicular from the vertex to the circular base is generally parallel to the pair of electrodes. In this case, the permittivity anisotropy xcex94xcex5 desirably has a value of       "LeftBracketingBar"          Δ      ⁢              xe2x80x83            ⁢      ϵ        "RightBracketingBar"     greater than             P      s                      ϵ        0            ·              (                  E          +                                                    P                s                            ⁢              cos              ⁢                              xe2x80x83                            ⁢              φ                                      ϵ              lc                                      )            ·              (                                            sin              2                        ⁢                          θ              ·              cos                        ⁢                          xe2x80x83                        ⁢                          δ              ·              cos                        ⁢                          xe2x80x83                        ⁢            φ                    +                                    sin              ⁢                              xe2x80x83                            ⁢              2              ⁢                              θ                ·                sin                            ⁢                              xe2x80x83                            ⁢              δ                                      2              ⁢              tan              ⁢                              xe2x80x83                            ⁢              φ                                      )            
where E is an amplitude of the exterior electric field, xcex51c is a permittivity of the liquid crystal layer in a same direction of the external electric field, xcfx86 is an azimuthal angle accomplished by a first segment and a second segment. The first segment is directed in a radius direction of the circular base and in parallel to the first pair of electrodes. The second segment links the other end of the liquid crystal molecule and a center of the circular base of the cone. xcex4 is an angle accomplished by neighboring two of the plurality of smectic layers.
The liquid crystal layer may be formed of a smectic liquid crystal material which is a DHF(Deformed Helix Ferroelectric) liquid crystal material.
The liquid crystal layer may be formed of a smectic liquid crystal material which is a polymer stabilized ferroelectric liquid crystal material.
The liquid crystal display may further include a first pair of electrodes between which the liquid crystal layer is located. In this case, the liquid crystal layer may have a chiral-smectic C phase. The liquid crystal molecule may be mono-stabilized in an initial state such that a first projection component to the first pair of electrodes of the long axis and a second projection component to the first pair of electrodes of a rotation axis of a cone drawn by the liquid crystal molecule are respectively equal to a orientation process direction of the first pair of electrodes.
The liquid crystal layer may consist of a ferroelectric liquid crystal material having an isotropic phase, a chiral-smectic phase, and a smectic C* phase and mono-stabilized in an initial state while a phase transition from the chiral smectic phase to the smectic C* phase is done.
In order to achieve another aspect of the present invention, a method of operating a liquid crystal display cluding a liquid crystal molecule is composed of:
applying an external electric field to the liquid crystal molecule;
generating a depolarization field opposite to the electric field by reversing a spontaneous polarization of the liquid crystal molecule to produce an effective electric field which is sum of the external electric field and the depolarization field;
applying to the liquid crystal molecule a torque generated by a coupling of a permittivity anisotropy of the liquid crystal molecules to the effective electric field; and
settling an orientation of the liquid crystal molecule, wherein the orientation is determined by an effective electric field.
In this case, the liquid crystal molecule desirably has a long axis, a short axis perpendicular to the long axis, a spontaneous polarization Ps along the short axis, a first permittivity xcex5// along the long molecular axis, a second permittivity xcex5xe2x8axa5 along the short axis. The second permittivity xcex5xe2x8axa5 is derived from polarizations other than the spontaneous polarization. Furthermore, it is desirable that a permittivity anisotropy factor xcex94xcex5 defined as xcex94xcex5= xcex5//xe2x88x92xcex5xe2x8axa5has a value of
xcex94xcex5 less than 0.
In order to achieve still another aspect of the invention, liquid crystal includes a liquid crystal molecule having a long axis, a short axis perpendicular to the long axis, a spontaneous polarization Ps along the short axis, a first permittivity xcex5// along the long molecular axis, and a second permittivity xcex5xe2x8axa5 along the short axis. Here, the second permittivity is derived from polarizations other than the spontaneous polarization. A permittivity anisotropy xcex94xcex5 defined as
xcex94xcex5=//xe2x88x92xcex5xe2x8axa5
has a value of xcex94xcex5 less than 0. A direction of the liquid crystal molecule is determined by an effective electric field which is a sum of an exterior electric field applied to the liquid crystal and a depolarization field caused by the spontaneous polarization.